Hylobatids, the lesser or smaller apes, differ from great apes by not building nests, being smaller-bodied, showing no size difference between genders, and consistently exhibiting ischial callosities. Adult white-cheeked gibbons exhibit dichromism (different color for each gender). Both genders are born a buff color. Juveniles turn black with a white cheek pattern between six months and a year. Females return to a buff hair color with a black crown patch when they become sexually mature. Males remain black with white cheek patches. Both sexes have very dense hair coat compared with other gibbon species living closer to the equator. Their face, palms and soles are bare and pigmented. Their elongated posture is a key adaptation for suspensory behavior. Their arms and hands are elongated and there is a deep cleft between the index finger and opposable thumb. This thumb is not used in brachiation (swinging by the arms from branch to branch), but is instead used to climb thick branches, groom, and manipulate food. They have a small throat sac used as a resonating chamber. Like all apes, they are tailless.

Listed as “critically endangered" on the IUCN Red List (International Union for Conservation of Nature and Natural Resources). Their population is estimated to be slightly greater than 1,000 individuals.